A surface light source device with an organic EL element can have a planar shape and provide light of white color or a color close thereto. Accordingly, the surface light source device with an organic EL element is thought to be employed, for example, as a light source of a lighting apparatus for lighting space in living environments or as a backlight unit used for display devices.
However, the currently known organic EL element is too low in efficiency to be employed for the aforementioned lighting use. Therefore, it is desired to improve the light extraction efficiency of the organic EL element. As a method for improving light extraction efficiency of the organic EL element, there is known a method wherein a variety of concavo-convex structures are formed on a side that is closer to a light output surface than the organic EL element. For example, there are proposals such as providing a prism containing a fluorescent compound (Patent Literature 1) and providing a microlens array (Patent Literature 2) on the light output surface. These concavo-convex structures can achieve favorable collection of light, to thereby improve light extraction efficiency.